Fuel systems for aircraft gas turbine engines

ABSTRACT

In an aircraft having a gas-turbine engine, one or more main jet nozzles directed downwardly to provide weight-supporting thrust for the aircraft, a plurality of stabilising nozzles, duct-means for feeding air at pressure from the engine compressor to the stabilising nozzles, and a pilot&#39;&#39;s adjustable valve means for adjusting air flow from the stabilising nozzles to control the attitude of the aircraft, the provision of a fuel system for the engine comprising, a source of fuel at pressure, a constant speed fuel flow control to feed fuel from the source to the engine for constant and slowly varying speeds, an acceleration fuel flow control to feed fuel from the source to the engine during engine acceleration at a rate ensuring safe acceleration without compressor stall, an auxiliary fuel flow control and control means for said auxiliary fuel flow control responsive to air flow to one or more stabilising nozzles to supply extra fuel flow to the engine during engine acceleration over and above the fuel flow supplied by the acceleration fuel flow control.

United States Patent 191 Robinson Jan.8,1974

[ FUEL SYSTEMS FOR AIRCRAFT GAS TURBINE ENGINES Keith Robinson,Churchdown, England [75] Inventor:

[73] Assignee: Dowty Fuel Systems Limited,

Cheltenham, England [22] Filed: Feb. 29, 1972 [211 App]. No.: 230,494

[30] Foreign Application Priority Data Primary ExaminerDuane A. RegerAssistant Examiner-Jesus D. Sotelo Att0mey-John W. Malley et al.

[ 5 7] ABSTRACT In an aircraft having a gas-turbine engine, one or moremain jet nozzles directed downwardly to provide weight-supporting thrustfor the aircraft, a plurality of stabilising nozzles, duct-means forfeeding air at pressure from the engine compressor to the stabilisingnozzles, and a pilots adjustable valve means for adjusting air flow fromthe stabilising nozzles to control the attitude of the aircraft, theprovision of a fuel system for the engine comprising, a source of fuelat pressure, a constant speed fuel flow control to feed fuel from thesource to the engine for constant and slowly varying speeds, anacceleration fuel flow control to feed fuel from the source to theengine during engine acceleration at a rate ensuring safe accelerationwithout compressor stall, an auxiliary fuel flow control and controlmeans for said auxiliary fuel flow control responsive to air flow to oneor more stabilising nozzles to supply extra fuel flow to the engineduring engine acceleration over and above the fuel flow supplied by theacceleration fuel flow control.

9 Claims, 6 Drawing Figures 1 FUEL SYSTEMS FOR AIRCRAFT GAS TURBINEENGINES This invention relates to an aircraft gas-turbine engine fuelsystem and more particularly it relates to a fuel system for agas-turbine engine of an aircraft having at least one main jet nozzledirected downwardly to provide weight supporting thrust for theaircraft, a plurality of stabilising nozzles, duct means for feeding airat pressure from the engine compressor to the stabilising nozzles, and apilots adjustable valve means for adjusting air flow from thestabilising nozzles to control the attitude of the aircraft. With anaircraft of this nature it is highly desirable that the pilot shouldhave the most effective control of the aircraft engine, and inparticular that the pilot should be able to accelerate the engine at itsmaximum rate at any instant during flight. The fuel system for agas-turbine engine is normally arranged so that acceleration fuel flowis provided to enable engine acceleration at a maximum rate just shortof the rate which could cause compressor stall. However, if during anacceleration of a gas-turbine engine a substantial flow of air is bledfrom the compressor delivery for feeding stabilising nozzles theoperating characteristics of the compressor are altered and a normalacceleration fuel flow control would give an acceleration considerablylower than normal acceleration. Such loss of engine acceleration reducesthe manoeuverability of the aircraft and could in some circumstancescause the aircraft to crash.

The object of the present invention is to provide a fuel flow controlsystem for a gas-turbine engine of an aircraft of the kind referred toin which the fuel system provides compensation for the loss ofacceleration that could otherwise take place when air from the enginecompressor is fed to stabilising nozzles.

In an aircraft having a gas-turbine engine, at least one main jet nozzledirected downwardly to provide weight supporting thrust for theaircraft, a plurality of stabilising nozzles, duct means for feeding airat pressure from the engine compressor to the stabilising nozzles, and apilot's adjustable valve means for adjusting air flow from thestabilising nozzles to control the attitude of the aircraft, the presentinvention comprises the provision ofa fuel system for the enginecomprising a source of fuel at pressure, a constant speed fuel flowcontrol to feed fuel from the source to the engine for constant andslowly varying speeds, an acceleration fuel flow control to feed fuelfrom the source to the engine during engine acceleration at a rateensuring safe acceleration without compressor stall, an auxiliary fuelflow control, and a control means for said auxiliary fuel flow controlresponsive to air flow to at least one stabilising nozzle to supplyextra fuel flow to the engine during engine acceleration over and abovethe fuel flow supplied by the acceleration fuel flow control.

Compensating means may be provided for said auxiliary fuel flow controlresponsive to compressor delivery pressure within the engine and/or toengine speed.

The auxiliary fuel flow control may be arranged to supply extra fuelflow in accordance with the air flow rate selected for the stabilisingnozzles.

The acceleration fuel flow control may comprise a main variable throttlethrough which fuel flows to the engine, first-adjustment means for themain throttle controlled by at least one engine air pressure, a by-passvalve connected to by-pass fuel from upstream of the throttle, andpressure drop means responsive to pressure drop at the throttle toadjust the by-pass valve so that the remaining fuel flow passing throughthe throttle to the engine gives a predetermined pressure drop at thethrottle.

The predetermined pressure drop may be arranged to vary in accordancewith the square of engine speed.

The auxiliary fuel flow control may comprise an auxiliary variablethrottle connected in parallel with the said main variable throttle, andauxiliary adjustment means for the auxiliary throttle responsive to theair flow rate within the duct means to adjust the auxiliary throttle sothat the extra fuel flow to the engine permitted by the auxiliarythrottle is in accordance with the air flow rate within the duct means.

One embodiment of the invention will now be described with reference tothe accompanying drawings, in which FIG. 1 is a diagrammatic elevationof an aircraft including a two-spool gas-turbine engine and stabilisingjets,

FIG. 2 is a diagrammatic plan view of the engine in FIG. 1 and thegallery system feeding the burners which forms part of the presentinvention,

FIG. 3 is a graph showing a desired relation between fuel pressure andflow rate obtained by the arrangement in FIG. 2, and

FIG. 4 is a diagram of the fuel system for feeding fuel to the engine ofFIGS. 1 and 2 which is shown in three parts joinable together, the partsbeing indicated as FIGS. 4A, 4B and 4C. Reference is made initially toFIG. 1 of the drawings. The aircraft is conventionally provided with afuselage 1, Wings 2 and 3, tail planes 4 and a rudder. The engine 5mounted in the aircraft is of the two-spool kind embodying a lowpressure compressor A, a high pressure compressor B, combustion chamberC, a high pressure turbine D and a low pressure turbine E. Thecompressor A, and turbine E are mounted on a single shaft for rotationtogether whilst the compressor B and turbine D are mounted on anothershaft for rotation together co-axially with compressor A and turbine E.The low pressure compressor delivers compressed air both to the highpressure compressor B and to a pair of swivelling nozzles 6 which may beadjusted to produce either horizontal or vertical thrust. The airdelivered by the high pressure compressor B enters the combustionchambers C and the high temperature gas leaving the combustion chamberspasses the high pressure turbine and the low pressure turbine insuccession. The exhaust gases from the low pressure turbine are fed to asecond pair of swivelling nozzles 7 adjustable with the nozzles 6 toprovide vertical or horizontal thrust.

A bleed connection 8 from the high pressure compressor B connects to aduct system comprising four ducts 9, 11 12 and 13 leading respectivelyto stabilising jets 14, l5, l6 and 17 located respectively at the twowing tips, the nose and the tail of the aircraft. The duct 13 is alsoconnected to a pair of opposed horizontal jets 18 mounted adjacent tothe aircraft rudder. The stabilising jets 14, I5, 17 and 18 are providedwith obturators connected for operation by the adjacent control surfaceson the wings, tail and rudder. The nose jet 16 is provided with anobturator also controlled by the pilots control leading to the tailcontrol surfaces. An aircraft of this nature is already known and oneexample I may be seen in U.S. Pat. No. 3,160,368.

in order to feed fuel to the combustion chambers, a fuel galleryarrangement is provided as diagrammatically illustrated in FIG. 2. Fuelenters the engine at a pipe 19 and passes to a pair of pressure reliefvalves 21 and 22, the valve 21 having a lower effective spring loadingthan the valve 22. The deliveries from valves 21 and 22 connectrespectively to a pair of circular galleries 23 and 24 encircling theengine at the location of the combustion chambers. A number ofvapourising burner assemblies 25 are provided around the engine feedingto the combustion chambers. Each burner assembly includes a vapourisingtube 26 fed with fuel from a swirl chamber 27. A connection 28 fromgallery 23 enters the swirl chamber tangentially and a second pipe 29from the gallery 24 enters the swirl chamber axially. The main purposeof the swirl chambers is to equalise flow to all burner assemblieswithout involving small diameter orifices which could become choked withsolid contaminants in the fuel. Fuel from the pipe 28 produces the basicswirl in the chamber 27 and fuel which enters from the pipe 29 is alsothereby swirled by virtue of the swirling fuel already within thechamber 27. The lower pressure loading of the valve 21 ensures that forlow flow rates fuel enters only through valve 21, gallery 23, pipes 28and the swirl chambers 27. When the fuel pressure reaches apredetermined value the valve 22 will open carrying further fuel intothe swirl chambers through galleries 24 and pipes 29. The main functionof the valves 21 and 22 is toprovide an almost linear relation-betweenfuel pressure in pipe 19 and the actual total flow rate of fuel into theengine. A simple swirl chamber device on its own would? produce asquare-law characteristic which over the range of fuel flows requiredfor the engine would produce an unacceptably high pressure at maximumfuel flow. Additionally the near linear relation between pressure andfuel flow rate is usefully employed within the fuel control system ofFlG..4 as will be described in order to assist in the stable governingof the engine.

Reference is now made to FIG. 4 of the accompanying drawings. Fuel isdelivered from the aircraft fuel tankin a conventional manner by means.of a boost pump (not shown) and an engine-driven gear pump 31. Fuelenters the pump at pipe 32 and leaves at pipe 33. A pressure reliefvalve 34 by-passes fuel above a predetermined maximum pressure from thedelivery pipe 33 through a pipe 35 and back into a boost pressure pipenetwork 36 which connects back to the pump inlet 32. The fuel from pipe33 initially enters an emergency change-over valve 37 which is capableof delivering the flow from pipe 33 into either the normal deiliverypipe 38 or the emergency delivery pipe 39. The controlling spool 41within valve 37 is servo-adjusted by means of a piston 42 slidablewithin a cylinder 43 and a vent valve 44 adjusted by a solenoid 45. Highpressure liquid from pipe 33 has access within the spool 41 to a centralpassage 46 from which fuel may flow both to the lower end of the spool41 and through a restrictor 47 to the cylinder 43. When valve 44 opensto vent cylinder 43 fuel at pressure acting on the lower end of thespool 41 will push the spool 41 upwardly to connect the delivery 33 tothe emergency pipe 39. Normally the valve 44 remains closed enablingpressure to exist in cylinder 43 to hold the spool 41 in its lowermostposition in which the delivery pipe 33 is connected to the pipe 38.

The pipe 38 connects to a filter 48 whose function is to deliver to pipe49 a small filtered flow of fuel suitable for servo use within the fuelsystem. The main delivery from pipe 38 however leaves filter 48 at pipe51 to enter the first throttle valve 52.

Within valve 52 the throttle element comprises a ho]- low piston 53slidable in a valve cylinder to control the opening between a port 54connected to the inlet pipe 51 and a port 55 connected to an output pipe56. Within the hollow piston, ports 57 permanently connect the hollowpiston to the port 54 whilst a throttle aperture 58 overlaps the port 55to form a variable throttle whose throttling effect is determined by theposition adopted by the piston 53 relative to the port 55. In order toadjust the piston 53 a servo piston 59 is secured to it for slidingmovement within a cylinder 61.

The servo supply pipe 49 connects directly to the upper end 62 of thecylinder 61 and indirectly through a restrictor 63 to the lower end 64of cylinder 61. It will be seen that the upper effective area of piston59 exposed to full pressure in the end 62 of the cylinder is smallerthan the lower area of piston 59 exposed to the end 64 of the cylinder.

The adjustment of the hollow piston 53 is by virtue of two separate ventcontrols acting on vent pipes 65 and 66. in order to ensure that theservo piston 59 can never be controlled by the joint effect ofsimultaneous venting of the pipes 65 and 66, a discriminator 67 is'provided. The discriminator comprises upper and lower chambers 68 and69 separated by a flexible diaphragm 71. The diaphragm 71 is normallyspringloaded against a vent 72 in chamber 69 such that if the diaphragm71 lifts from vent 72 the lower chamber 69 is connected to the lowpressure-pipe 36 via passage 73. The vent pipe 65 and the lower end 64of cylinder 61 are directly connected to the chamber 69. The upperchamber 68 is directly connected to vent passage 66 and also through arestrictor 74 to the upper end 62 of cylinder 61.

If the vent pipe 65 is vented, the pressure is reduced in the lowerchamber 69 and in the lower end 64 of cylinder 61 and the diaphragm 71closes vent 72. On the other hand if the vent pipe 65 is closed but pipe66 is vented, the diaphragm 71 will lift to open the vent 72 a smallamount which will allow a controlled reduced pressure in accordance withthe reduced pressure in vent 66 to be generated in lower chamber 69 anddirectly fed to the lower end 64 of cylinder 61. Thus either of thevents 65 or 66 may generate a controlling pressure at the lower end 64of cylinder 61.

In order to control the vent pipe 66 the upper part of the throttlevalve 52 is provided with a chamber 75 within which a lever 76 ispivotally mounted at a fulcrum 77. The lever 76 carries a conventionalhalf-ball valve 78 controlling flow from the vent pipe 66 into thechamber 75 which is in turn connected to the low pressure network 36.Within chamber 75 a bellows assembly is provided comprising threebellows 79, 81 and 82 and an inter-connecting yoke 83 which bearsthrough pin 84 on to the lever 76. The bellows 79 is connected to a pipe85 and receives a signal derived from the high pressure (P3) deliveredby the high pressure compressor B. The bellows 81 is connected to a pipe86 and receives a signal of the delivery pressure of the low pressurecompressor A (P2). The bellows 82 is evacuated. The bellows are mountedby being secured within the chamber 75 to the yoke 83 so that the forceexerted on the lever is a function of the difference of the pressure P2and the pressure derived from P3 and forms part of an acceleration fuelflow controlling system.

Also within the chamber 75 a feed-back spring 88 is provided, beigcompressible by movement of the throttle piston 53 so as to exert on thelever 76 a force dependent on the position of piston 53. An adjuster 89is provided for pre-set datum adjustment of the loading of the spring byadjusting the load in a spring 87 acting on lever 76 in opposition tospring 88.

The thrust exerted by the pin 84 on lever 76 determines the escape flowpermitted from the vent 66 into chamber 75. Variation of the pressuresignal on the pin 84 will alter the position of the lever 76 which willin turn alter the pressure at the lower end 64 of cylinder 61 to movepiston 59 and piston 53 in the sense to alter the compression of spring88 so that the changed force exerted by the spring will compensate forthe change of force exerted by the pin 84 thereby restoring the halfballvalve 78 to a predetermined clearance relative to the vent 66. In otherwords the position of the piston 53 will be in proportion to the forceexerted by the pin 84 on the lever 76. The described positional controlfor the throttle piston 53 by virtue of the lever 76 will only beeffective provided that the vent 65 is closed. The description of thecontrol for vent 65 will appear further in this specification.

The control of fuel flow by the throttle piston 53 is made fullyeffective by providing a by-pass valve 91 which operates to control aby-pass flow of fuel from the pipe 38 through a by-pass pipe 92 and alow pressure relief valve 93 into the low pressure network 36. Theby-pass valve is basically formed by a hollow piston 94 which controlsthe flow between a pair of ports 95 and 96 in a co-operating cylinder. Aport 97 in the hollow piston connects permanently to the port 95 duringpiston movement and a throttle aperture 98 in overlapping relation withthe port 96 determines an adjustable throttle controlling the flow offuel through the by-pass pipe 92 and valve 93 into the low pressurenetwork 36. Axial adjusting movement of the piston 94 is provided by twoopposed signals. The first of these signals is a pressure drop signalbetween two differing pressures existing in a chamber 99 enclosing theupper end of piston 94 and in a chamber 101 enclosing the lower end ofpiston 94. The chamber 101 is connected by pipe 102 to the port 55 inthe throttle valve 52 and carries the pressure downstream of thethrottle valve 52. The upper chamber 99 receives pressure from a pipe103 connected to the pipe 51 upstream of the throttle valve 52.

For the purpose of adjustment in setting the system up for operation,means are provided for adjusting the relationship between the pressuredrop at throttle 52 and the downward force acting on the hollow piston94, these means comprising a variable restrictor 104 and a fixedrestrictor 105 connected in series between the pipes 102 and 103, thespace 99 being connected to the junction between the restrictors 104 and105. In effect the restrictors 104 and 105 form a pre-set potentiometerby which a predetermined portion of the pressure drop between the pipes102 and 103 may be applied in the space 99 whereby a downward force willact on piston 94 in proportion to the pressure drop occurring atthrottle valve 52.

In the lower part of the housing for the by-pass valve 91 a rotary driveshaft 106 is suitably mounted in bearings and is adapted to be driven bythe high pressure spool of the engine. The shaft 106 enters the chamber101 and carries a pair of gears 107 and 108. The gear 108 is ofsubstantial axial length and meshes with a gear 109 to rotate piston 94to reduce its friction to axial movement. The gear 109 has a largernumber of teeth than the gear 108 so that piston 94 rotates at acomparatively slow speed. The gear 107 meshes with a gear 111 to rotatea carrier 112 in a lower chamber 110. On the carrier 112 a set offlyweights 113 are pivotally mounted for centrifuging movement. Theflyweights 'are so constructed that their overall density is about twicethe mean density of the range of fuels to be used. The flyweights may bemade of metal with internal completely closed cavities to provide thecorrect overall density. The lower chamber is connected to chamber 101and by virtue of its pipe connections receives a flow of fuel which atany instant can be said to have the same density as the fuel flowing tothe engine. The flyweights will cause rotation of fuel in chamber 110and the centrifugal force generated on the flyweights will beproportional to the difference in overall density of the flyweights anddensity of the fuel such that with reducing fuel density the centrifugalforce becomes higher and vice versa.

The flyweights 113 are provided with inwardly directed levers 114 toexert an axial thrust on a central cup 115. The cup 115 contains aspring 116 which reacts against a push rod 117 extending from the piston94. The spring 116 and the cup 115 are so arranged that at low enginespeeds the centrifugal force generated by the flyweights 113 cannotovercome the load of spring 1 16 and the flyweights are held againstinward stops on the carrier by the spring load. The spring force thenacts as a minimum force on piston 94. At higher speeds when thecentrifugal force is higher than the load of spring 116the cup 115 makesdirect contact with the rod 117 so that centrifugal thrust istransmitted directly to the piston 94 in opposition to the pressure dropforce exerted on piston 94 by virtue of the pressures in chambers 99 and101. It will be appreciated that the flyweights 113 must be capable of asubstantial range of radial movement in order to effect control movementof the hollow piston 94. The ends of the control levers 114 which reacton the hollow cup 115 are so shaped that as the flyweights 113 moveoutwardly the mechanical leverage of levers 114 is reduced thus toensure that the thrust exerted by the flyweights 113 on the hollowpiston 94 is always in substantially constant proportion to the squareof the rotational speed of the high pressure spool in the engine for anyfixed value of fuel density at the flyweights.

The operation of the by-pass valve 91 is to ensure that the pressuredrop occurring at the throttle valve 52 is in proportion to thecentrifugal force exerted by the flyweights 113. The square-lawcharacteristic between pressure drop and flow at the throttle aperture58 will then ensure that the mass flow rate of fuel passing through thethrottle aperture 58 is directly proportional to the speed of the highpressure spool for any one setting of the throttle 58 independently ofnormal fuel density changes, the surplus fuel being by-passed from pipe38 through pipe 92 and the adjustable throttle aperture 98 back to thelow pressure network 36. The function of the spring 116 at low enginespeeds is to ensure a small excess of fuel at light-up by providing aminimum force acting on piston '94 in opposition to the pressure dropforce. Over the range of normal running speeds, the cup 115 willdirectly contact the rod 117 ensuring that the centrifugal force fromflyweights 113 is directly transmitted to piston 94..

The acceleration fuel flow determined by the throttle 52 is in partdetermined by the pressure drop function applied by the by-pass valve 91and in part by the setting of the piston 53 by virtue of the bellowsassembly 79, 81, 82. The throttle aperture 58 may have a suitableshaping to ensure that acceleration fuel flow under all conditions ofoperation is just below the value which could cause compressor stall.

Acceleration fuel metering by the throttle 52 takes place only underconditions when the vent 65 is closed. The vent 65 extends to themechanical governor unit 121. Within the governor unit 121 a chamber 122is provided within which a lever 123 is pivotally mounted at a fulcrum120. A number of forces are arranged to act on the lever 121, theprincipal being the centrifugal force given by a set of flyweights 124which are rotatably driven by a shaft 125 from the low pressure spool inthe engine. The flyweights 124 are of well-known kind which provideinsensitivity of output force to any variation in density of liquidwithin which they rotate. As is usual with aircraft engine fuel controlsystems the flyweights are very conveniently submerged in fuel so thatinsensitivity to variation in fuel density is obtained. The centrifugalforce due to flyweight rotation is transmitted through a rod 126 to acton the lever 123. An adjustable compression spring 127 acts in theopposite direction on lever 123, the loading of this spring beingadjusted by a cam 128 which is mechanically connected for adjustment bythe pilots control lever 129. The lever 123 acts on a half-ball valve131 which controls the escape flow from the vent pipe 65. It will beseen that fuel escaping from the vent pipe 65 enters the chamber 122from which it will have access to the flyweights 124 and also will beable to escape to the low pressure pipe network 36. The lever 123 andhalf-ball valve aresoarranged that a very small movement (approximately.002 inches) is necessary at the halfball valve to adjust the vent pipe65 between the fully closed and the fully open position. This ensuresthat the range of centrifugal movement of the flyweights 124 is kept toan absolute minimum to assist in accurate space governing. When thehalf-ball valve 131 opens slightly to provide governing control, thevented fuel from the lower chamber 69 in the throttle valve 52 willreduce the pressure in chamber 69 and the lower end 64 of cylinder 61thus causing diaphragm 71 to close vent 72. When vent 72 is closed thecontrol of movement of the piston 53 is entirely under the control ofthe half-ball valve 131. The fact that the pressure drop across thethrottle 52 is controlled to the square of high pressure spool speed isthen of no consequence since the pressure drop is only able to alter asmall amount compared with the large change in area of the orifice 58resulting from movement of half-ball valve 131. lnaccuracy of themechanical governor 121 with variation in altitude of the aircraft canoccur as a result of the very substantial change in fuel flow withaltitude to maintain a particular engine speed. With increase inaltitude the fuel required to maintain a particular engine speed willreduce very substantially. The pressure of fuel supplied to the engineat the pipe 19 will depend on the: flow rate of fuel to the engine inaccordance with the graph of FIG. 3 and this pressure variation withvariation in fuel flow is reacted on the lever 123 through the half-ballvalve 131 with the result that with increase in altitude a desired speedselected on the pilots lever 129 will creep upwardly. This tendency iscompensated by a creep control which exerts a compensating force onlever 123. A restrictor potentiometer 130 is fed with low pressuredelivery pressure (P2) from the engine and it delivers into the pipe 132a fixed proportion of this pressure. This pressure is fed to a bellows133 which reacts on an auxiliary lever 134. The bellows 133 is opposedby an evacuated bellows 135 to ensure that the force acting on lever 134remains in proportion to the absolute value of pressure in pipe 132. Thefulcrum 136 of the lever 134 is adjustable to vary the point at whichthe lever 134 engages lever 123 relative to its fulcrum 120. Thus withincrease in altitude the reduced fuel pressure force acting on thehalf-ball valve 131 is compensated by reduction in the thrust exerted bythe bellows 133 on the lever 123, thus compensating the governor creep.

An emergency protection must be provided to protect the engine in thecase of excessive turbine temperatures and it is necessary whenever anexcessive temperature occurs for quick action to be taken to reduce fuelflow. In the present fuel system this is conveniently effected throughthe mechanical governor 121. An electric thermocouple temperaturesensing device for the jet pipe of the engine is arranged to operatethrough an amplifier to generate an electric signal dependent on jetpipe temperature whichis connected to an electric force motor 137 toexert a corresponding torque on a lever 138. The torque exerted on lever138 is quite small and it is amplified by a hydraulic amplifier devicecomprising opposed jets 139 and 141 and opposed mechanically-connectedservo-pistons 142 and 143. The lever 138 extends as a flapper in betweenthe jets 139 and 141 and varies the relative escape flows from thesejets. The jets 139 and 141 receive operating fuel from the servo supplypipe 49 and the relative escape flows at the jets 139 and 141 will, byvirtue of restrictors, generate pressure which act oppositely on thepistons 142 and 143. Spring 144 feeds back the movement of pistons 142and 143 on to the lever 138 so that the movement of the pistons 142 and143 is in accordance with the torque exerted by the motor 137. Thepistons 142 and 143 react together through a spring 145 on to thecontrol lever 123 in such manner that, on the occurrence of excess jetpipe temperature, the spring 145 will exert an extra force on the lever123 to lift the halfball valve 131 from the vent 65 to move piston 53 toreduce the throttle aperture 58 and thus reduce fuel flow to the engine.

Another safety precaution necessary with the engine is the prevention ofexcessive pressure within the combustion chambers. This pressure is thepressure P3 delivered by the high pressure compressor. For this purposea pressure limiter unit 146 is provided. The pressure P3 enters the unitthrough a filter 147 into a chamber 148 where the pressure reacts on abellows 149 connected to receive ambient atmospheric pressure (P This isthe pressure which exists in the engine nacelle. The bellows 149 isconnected to operate on a lever 15] which is pivotally of flexurallymounted at 152. The pressure P3 acting within the chamber 148 escapesfrom the chamber through an orifice 153 which is connected in serieswith a restrictor 154 connected back to P pressure. The junction of theorifree 153 and restrictor 154 is connected to pipe 85. The

bellows 149 is arranged to compress when the pressure P3 reaches anexcessive value to move lever 151 to a position where it interrupts theflow of air into the orifice 153. Such interruption will reduce thepressure in pipe 85 which will reduce the force exerted by bellows 79 inthe throttle 52. This reduction in bellows force will upset the balanceof the lever 76 and cause movement of throttle piston 53 in the sense toreduce the throttle aperture 58 and thus reduce the fuel flow to theengine in an overriding manner.

The stabilising jets 14, 15, 16, 17 and 18 of the aircraft shown in FIG.1 require the flow of high pressure air from the high pressurecompressor and when the aircraft is in its vertical flight mode i.e. thenozzles 6 and 7 are pointed downwardly, the flow of stabilising air willcause a change in the engine operating conditions which will requirealtered fuel flow to the engine. There are two states of engineoperation to be considered, i.e. engine acceleration operation andengine constant speed operation. During engine acceleration the flow ofair from the high pressure compressor to the stabilising jets will meanthat a smaller quantity of air at a somewhat lower pressure is fed intothe combustion chambers. Clearly if the engine is to maintain the samevertical thrust from the jet nozzles 7 more fuel must be burnt in thereduced quantity of air entering the combustion chambers. To compensatethe fuel system to provide such extra fuel the reset unit 155 isprovided. Basically the unit comprises a throttle piston 156 adjustablymovable in a cylinder through a pair of spaced ports 157 and 158, athrottle aperture 159 within the piston providing a variable connectionbetween the ports depending on the position of the piston. The port 157connects through pipe 161 to the pipe 51 upstream of the throttle 52.The port 158 connects through pipe 162 to the pipe 56 downstream of thethrottle 52. Effectively the unit 155 provides a separate throttle inparallel with the throttle valve 52, which since the pressure dropacross the throttle 52 is controlled by the by-pass valve 91 will ensurean extra fuel flow in accordance with the setting of the piston 156. Thepiston 156 is adjusted by the control signals applied to a pair ofopposed bellows 163 and 164. Bellows 163 receives pressure P3 from thehigh pressure compressor and the bellows 164 receives the generalpressure PD existing in the ducts 9, 11, 12 and 13. This pressure PDwill vary in accordance with the flow rate of high pressure air to thestabilising nozzles. The two bellows 163 and 164 are connected togetherby a yoke 165 which reacts at a pin 166 on a lever 167 mounted in achamber 168. The lever is pivoted at a fulcrum 169 and carried ahalf-ball valve 160 which controls escape flow from a vent 170. Aservo-piston 171 secured to the throttle piston 156 moves within acylinder having upper and lower chambers 172 and 173. In the upperchamber 172 servo fuel from pipe 49 acts on a small area of piston 171.A restrictor 174 feeds liquid at pressure to the lower chamber 173 wherethe pressure will act on a larger area of piston 171. The vent 170 isalso connected to the chamber 173 and escape flow from the vent 170 willdetermine a lower pressure within the lower chamber 173. Movement of thehalf-ball valve 160 to increase escape flow from the vent 170 willreduce pressure in the lower chamber 173 causing the servo-piston 171 tomove downwardly. The resulting movement of the piston 156 is fed back onto the lever 167 by means of a spring 175 so that movement of piston 156as a result of alteration of the position of the half-ball valve willrestore the half-ball valve 171 into an equilibrium position where theescape flow is such that there is a force balance on piston 171 as aresult of the difference pressures in chambers 172 and 173. Theselection of air flow in any of the ducts to the stabilising nozzleswill cause a reduction in the pressure PD fed to the bellows 164 whichwill cause lift of the half-ball valve 160 to reduce pressure in chamber173. Downward movement of the piston 156 will increase the size ofaperture 159 to the ports 157 and 158 thus permitting an increase infuel flow into the pipe 56. The shaping of the throttle aperture 159ensures that the increased fuel fed to the engine is in accordance withthe reduction of pressure PD which in turn is in accordance with theactual flow rate of air fed to the stabilising nozzles. By this meansthe acceleration rate of the engine may be kept as high as possibleirrespectively of the amount of air fed to the stabilising nozzles. Theuse of compressor delivery pressure P3 as part of the signal control forextra fuel flow ensures a degree of altitude compensation of the extrafuel flow. Also the flyweights 113 will operate to control pressure dropat aperture 159 as at aperture 58 ensuring speed compensation for theextra fuel flow.

The permitted increase in fuel flow to the engine by operation of theunit 155 when air flow is selected for any stabilising jet operatesprincipally to increase fuel flow during the acceleration mode of theengine. To a lesser degree however, the opening of the throttle aperture159 following demand for stabilising jet air flow during constant speedoperation of the engine will also involve a slight resetting of themechanical governor 121. Since the mechanical governor 121 actsoverridingly to control the position of the throttle piston 53 duringconstant speed operation of the engine, the opening of the extrathrottle passage 159 when air flows from the stabilising jets willitself increase fuel flow to the engine which will initially react onthe governor 121 as an increased pressure at half-ball valve 131. Enginespeed will change both as a result of increased fuel flow and of thebleed off of high pressure air, and the governor 121 will then readjustthe throttle valve 52 to give a slightly lower fuel flow rate. Theoverall effect will be a slight reduction of the constant engine speedand a slightly increased total fuel flow to the engine.

The throttle unit 52 is the first throttle unit which assists indetermination of fuel flow to the engine. Fuel leaving the firstthrottle unit 52 then passes through the speed throttle unit 176 beforepassing to the engine. The second throttle unit performs the dualfunction of a shut-off valve and a fuel scheduling device. The throttleunit 176 comprises a hollow piston 177 movable in a cylinder 178 tocontrol the relative flows at four ports 179, 181, 182 and 183. The pipe56 connects to the port 182 and the pipe 19 for the engine connects tothe port 179. The upper and lower ends of the cylinder 178 are connectedto the low pressure network 36 so that no hydraulic pressure acts in theendwise sense on the throttle piston 177. The port 183 also connects tothe low pressure network 36. The post 181 connects to a pipe 184 tocarry fuel by-passed from the pipe 56 during the controlling functionexerted by the throttle valve 176.

The throttle piston 177 is movable in the endwise sense by means of alever 185 mechanically connected for movement by the pilots controllever 129. The cam 128 and the lever 185 are moved together by'thepilots lever 129. Within the throttle piston 177 there are three ports186, 187 and 188 which connect to the hollow interior of the piston. Theport 186 over the whole range of movement of piston 177 makes anunthrottled connection with port 182. In the shut-off position of thethrottle piston 177 (i.e. its uppermost position) port 186 forms asubstantially unrestricted connection between the ports 182 and 183 tobypass. all delivered fuel back to the low pressure network 36. In thefuel scheduling position of the throttle 177 corresponding to themovement of the pilots lever 129 between idle and full speed (the rangeof movement of the piston 177 except for its uppermost position) theport 188 will make a variable throttled connection with the port 179 andthe port 187 will make a substantially unthrottled connection with theport 181. Thus over the operating range from idle to full speed, fuelflow to the engine passes from pipe 56 through ports 182 and 186 intothe hollow interior of piston 177 and then leaves through two paths, onecomprising the throttle aperture 188 in the port 179 to feed to theengine, and the other comprising the port 187 whereby fuel may passthrough port 181 and 184 to the by-pass valve 189. In the shutoffposition of the valve 176 the port 188 will be closed by the landbetween ports 181 and 179 and the port 187 will connect to the port 182.

A second by-pass valve 189 co-operates with the sec ond throttle valve176. The by-pass valve comprises a flexible diaphragm 191 mounted inbetween upper and lower chambers 192 and 193 in the unit 189. The lowerchamber 193 is connected directly to the pipe 184 and the upper chamber192 is connected directly to the port 181 via pipe 194. Thus thepressure drop occurring between port 182 and port 179 due to the enginefuel flow through the throttle aperture 188 will be fed to act onopposite sides of the diaphragm 191 to adjust its position against aspring 195. The diaphragm 191 controls the by-pass flow of fuel from thechamber 193 through a port 196, such by-pass flow from port 196 enteringthe low pressure network 36. The action of the diaphragm 191 thereforeis to control pressure drop occurring between ports 182 and 179 in thethrottle unit 176 to a substantially constant value which is inaccordance with the loading of the spring 195. Under some circumstancesof operation the fuel flow through the throttle unit 176 will not besufficient to produce a pressure drop which will open the by-pass valve189, in which case all the fuel entering the second throttle unit 176will leave through the pipe 19 to the engine.

For the purpose ofproviding a minimum idling flow of fuel to the engine,the by-pass valve 189 is provided with an adjustable restrictor 197 tocarry fuel between the pipes 194 and 184 parallel with the flow pathpresented by the throttle aperture 188. The restrictor 197 then providesa minimum value for the throttle aperture 188 to maintain a minimum fuelflow to the engine under idling conditions. In the shut-off position forthe throttle valve 176 the port 181 will be isolated since the port 187is then in register with port 182 and therefore all flow to the engineis cut off. Also the by-pass port 181 is closed. 1

Convention means for water injection may be provided in the engine onoperation of a suitable control bythe pilot in order to lower turbinetemperatures for short periods during high power operation. The meansfor water injection concern the present invention in so far that thefuel system must be capable of adjustment whilst water is injected intothe engine to supply a greater controlled fuel flow. The water injectionsystem for the engine includes a signalling switch operated as a resultof water injection to the engine, such switch being arranged to energisea pair of solenoids 198 and 199 in the fuel system. The solenoid 198when energised will open a valve 201 between two pipes 202 and 203. Thepipe 202 connects to thepipe 19 delivering fuel to the engine whilst thepipe 203 extends to an upper auxiliary valve section 204 of throttleunit 176. Valve section 204 is really a shut-off valve which opensconnection between the pipe 203 and a pipe 205 over the whole range ofmovement of the pilots throttle 129 with the exception that theconnection is cut off when the control handle reaches the shut-offposition. The pipe 205 is connected to the servo supply pipes 49 whichin turn connects back to the filter 48 at the junction of pipes 38 and51. Therefore when the solenoid 198 is energised to open valve 201 aconnection is opened from the pipe 51 to the pipe 19 providing a bypasscircuit for the two throttle valves 52 and 176. The flow rate of fuel inthis by-pass circuit is determined firstly by the total pressure dropcontrolled across the two throttle valves 52 and 176 and the restrictiveeffect of the valve 201. The valve 201 is particularly concerned withoperational conditions during engine acceleration with water injection.The valve 199 is also energised during water injection and operates toload a spring 200 on to the lever 123 of the mechanical governor in thesense to raise the governed speed of the engine and thereby cause morefuel to flow to the engine. Water injection is used solely during lowlever flying, principally during take-off, to enable the maximum amountof engine pwer to be obtained. The water injection permits more fuel tobe injected into the engine, the cooling effect of the water thenpreventing excessive rise of turbine temperature.

For normal engine accelerations the pilots lever 129 is moved to selectan increased speed, such movement jointly increasing the load on thegovernor spring 127 and increasing the effective size of throttleaperture 188. Increasing the load of the governor spring will closehalf-ball valve 131 on to the vent 65. The servopiston 59 controllingthe throttle piston 53 is then under the control of the half-ball valve78 reacting through the diaphragm 71 of the discriminator 67. Theservo-piston 59 will then move piston 53 to a position determined by theengine gas pressures reacting on the bellows 79 and 81 and the by-passvalve 91 will operate to control by-pass in accordance with the squareof speed of the high pressure spool. Thus the fuel flow through thethrottle S2 is sufficient for safe engine acceleration. The engine willthen accelerate until the fuel flow is limited either by the throttlevalve 178 or by the mechanical governor. If the selected position of thepilots lever 129 is less than about percent of the full speed position,it is more likely that the flow limit of the throttle 176 will beattained before the mechanical governor reaches a controlling speed. Thefuel flow to the engine is then limited when the pressure drop of fuelflow through the throttle aperture 188 reaches a value which overcomesthe pressure loading of spring on diaphragm 191 at which point thediaphragm will lift from the vent 196 to allow by-pas s from the fueldelivered to valve 176 so that the flow leaving valve 176 through pipe19 is sufficient only to produce the fixed pressure drop determined bythe by-pass valve 189. By-passed fuel goes back to the low pressurenetwork 36. If however, the pilots lever 129 is set at a higher positionfor example above 80 percent of the maximum position it is more likelythat the engine speed will be limited by the mechanical governor in thatthe centrifugal force generated by flyweights 124 on rod 126 will beginto overcome the loading of the spring 127 thus lifting the half-ballvalve 131 from vent 65. When this happens the slightly lowered pressurein the lower chamber 69 of throttle valve 52 will cause the flexiblediaphragm 71 to close the vent 72 so that the reduced pressuredetermined in the vent pipe 65 will act solely in the lower end 64 ofservo cylinder 61 to control the position of the throttle piston 53.Since the pressure drop across the throttle valve 52 is capable only ofslow variation, the action of the flyweights acting aginst the spring127 will control engine speed accurately by determining the size of thethrottling aperture 58 in which fuel flows to the engine.

The arrangement of the two throttle valves 52 and 176 is such that forall engine speeds above about 85 percent of maximum speed the mechanicalgovernor must take control, the fuel flow permitted by the mechanicalgovernor when it passes through the throttling aperture 188 of throttlevalve 176 not producing a sufficient pressure drop to open the by-passvalve 189.

For selected lever positions below 85 percent the ultimate control ofthe engine will be either by the throttle valve 176 or by the mechanicalgovernor, depending on the altitude of operation and the resulting fuelflow necessary to maintain a selected speed. For quite low selectedspeeds the throttle valve 176 will normally effect control since the cam128 is so arranged that for low percentage positions of the controllever 129 the effective speeds selected by the mechanical governor isalways such as to demand a greater fuel flow to the engine than would bepermitted by the setting of the throttle valve 176 for that position oflever 129. There is however, a range of positions for the control lever129 lying between about 60 percent and 85 percent of the full speedposition in which either the throttle 176 or the mechanical governor 121will control engine speed in the constant speed state, this depending onthe alitutde of flight. With increase in altitude the fuel flownecessary to maintain a particular constant speed of the engine willbecome lower and therefore with increase in altitude the range of speedsover which the mechanical governor will control will extend more andmore from the 85 percent lever position back towards the 60 percentlever position. It will however be seen that engine speed is smoothlyadjustable by movement of the control lever 129 quite irrespectively ofthe altitude of flight. More particularly, however, the upper speedrange above about 85 percent is always under accurate governor control.Since the creep of the governor is compensated by the creep controldevice 130, 133, 135, the full speed position of the lever 129 controlsthe maximum safe speeds for the engine under normal operatingconditions. Under water injection operation it is true that themechanical governor would be set to a higher speed but the higher speedis then permissible because of the temperature reducing effect of thewater on the turbine.

The flyweights 124 in the mechanical governor may be driven by the highor the low pressure spool in the two-spool engine but as describedflyweights 124 are driven by the low pressure spool.

An emergency system is provided for the fuel system which comprises thechange-over valve 37, the emergency throttle unit 206 and the emergencyby-pass valve 207. The emergency throttle valve 206 comprises a hollowthrottle piston 208 arranged to control flow between a pair of ports 209and 211. The hollow piston 208 includes a port 212 constantly inconnection with the port 211 during piston movement and a throttleaperture 213 in overlapping relation with the port 209 so that itseffective throttle aperture is varied during piston movement. The pistonis adjusted in the axial direction by means ofa lever 214 which ismechanically connected for movement with the pilots lever 129.

The by-pass valve 207 includes an upper and a lower chamber 216 and 217separated by a flexible diaphragm 218 loaded by spring 219 on to a vent221 connected to the low pressure network 36.

The port 209 in the emergency throttle 206 connects to a pipe 222 whichextends to connect into the engine pipe 19. A non-return valve 223 islocated in the pipe 19 between the throttle valve 176 and the junctionof pipe 19 with pipe 222.

If under any circumstances of operation it appears to the pilot that thefuel system is not operating correctly, the pilot will press anemergency button to energise the emergency solenoid 45 to open valve 44and cause the change-over valve 37 to connect pump delivery 33 to theemergency pipe 39. From the emergency pipe 39 fuel enters the port 211of the emergency throttle and enters the hollow piston 208 through port212. Fuel leaves through the throttle aperture 213 into a port 209 andpipe 222. The pressure drop occurring at the throttle aperture isconnected through pipe 39 to the lower chamber 217 and through pipe 224to the upper chamber 216 of the emergency by-pass 207 and if thepressure drop is sufficient to overcome the loading of the spring 219the flexible diaphragm 218 will move to open the seat 221 so that fueldelivered from the emergency pipe 39 may pass into the low pressurenetwork 36, the remaining fuel leaving the emergency throttle being justsufficient to maintain a pressure drop at the throttle aperture 213 tohold the diaphragm 218 in its by-passing condition. The emergency systemcomprises principally the simple flow control formed by the emergencythrottle and the emergency by-pass and the fuel flow permitted to theengine by this emergency system is arranged to be about the same as thefuel flow permitted to the engine by the second throttle 176. Onswitching over to the emergency system the pilot will lose the variousrefined controls for his engine and clearly must adjust the emergencythrottle very carefully in order to adjust engine speed. Nevertheless,the emergency throttle 208 and emergency by-pass 207 represent a simplebut effective fuel control system. The non-return valve 223 is essentialto prevent metered fuel from the emergency system from leaking back intothe normal system.

I claim:

1. In an aircraft having a gas-turbine engine, at least one main jetnozzle directed downwardly to provide weight-supporting thrust for theaircraft, a plurality of stabilising nozzles, duct-means for feeding airat pressure from the engine compressor to the stabilising nozzles, and apilots adjustable valve means for adjusting air flow from thestabilising nozzles to control the attitude of the aircraft, theprovision of a fuel system for the engine comprising, a source of fuelat pressure, a constant speed fuel flow control to feed fuel from thesource to the engine for constant and slowly varying speeds, anacceleration fuel flow control to feed fuel from the source to theengine during engine acceleration at a rate ensuring safe accelerationwithout compressor stall, an auxiliary fuel flow control and controlmeans for said auxiliary fuel flow control responsive to air flow to atleast one stabilizing nozzle to supply extra fuel flow to the engineduring engine acceleration over and above the fuel flow supplied by theacceleration fuel flow control.

2. A fuel system as claimed in claim 1 including compensating means forsaid auxiliary fuel flow control responsive to compressor deliverypressure.

3. A fuel system as claimed in claim 2 wherein said compensating meansis also responsive to engine speed.

4. A fuel system as claimed in claim 1 wherein said control means forsaid auxiliary fuel flow control includes a proportional device arrangedto supply fuel flow in accordance with the air flow rate selected forthe stabilising nozzles.

5. A fuel system as claimed in claim 1 wherein the acceleration fuelflow control comprises a main variable throttle through which fuel flowsto the engine, first adjustment means for the main throttle controlledby at least one engine air pressure, a by-pass valve connected toby-pass fuel from upstream of the thorttle, and pressure drop meansresponsive to pressure drop at the throttle to adjust the by-pass valveso that the remaining fuel flow passing through the throttle to theengine gives a predetermined pressure drop at the throttle.

6. A fuel system as claimed in claim 5 wherein the said pressure dropmeans includes engine driven flyweights arranged to apply a forceproportional to the square of engine speed to the pressure drop means toensure that fuel'flow to the engine through the main throttle generatesa pressure drop proportional to the square of enginespeed.

7. A fuel system as claimed in claim 6 wherein the auxiliary fuel flowcontrol comprises an auxiliary variable throttle connected in parallelwith the said main variable throttle, and auxiliary adjustment means forthe auxiliary throttle responsive to the air flow rate within the ductmeans to adjust the auxiliary throttle so that the extra fuel flow tothe engine permitted by the auxiliary throttle is in accordance with theair flow rate within the duct means.

8. A fuel system as claimed in claim 7 wherein the said means responsiveto air flow within the duct means comprises a pair of opposed bellows,one connected to receive engine compressor delivery pressure and theother connected to receive pressure from the duct means whereby thebellows generates a force proportional to pressure drop between theengine compressor and the duct means, and servo means for adjusting theauxiliary throttle in accordance with said force.

9. A'fuel system as claimed in claim 7 including an engine-drivenflyweight device driven at a speed proportional to engine speed,manually-adjustable spring means acting on the flyweight device tooppose centrif ugal movement, manually-operable means for adjusting theload of the spring means, second adjustment means for the main throttleresponsive to centrifugal movement of the flyweight device,discriminator means responsive to the first and the second adjustmentmeans to connectthe adjustment to adjust the main throttle which willresult in the lower fuel flow to the engine.

2. A fuel system as claimed in claim 1 including compensating means forsaid auxiliary fuel flow control responsive to compressor deliverypressure.
 3. A fuel system as claimed in claim 2 wherein saidcompensating means is also responsive to engine speed.
 4. A fuel systemas claimed in claim 1 wherein said control means for said auxiliary fuelflow control includes a proportional device arranged to supply fuel flowin accordance with the air flow rate selected for the stabilisingnozzles.
 5. A fuel system as claimed in claim 1 wherein the accelerationfuel flow control comprises a main variable throttle through which fuelflows to the engine, first adjustment means for the main throttlecontrolled by at least one engine air pressure, a by-pass valveconnected to by-pass fuel from upstream of the thorttle, and pressuredrop means responsive to pressure drop at the throttle to adjust theby-pass valve so that the remaining fuel flow passing through thethrottle to the engine gives a predetermined pressure drop at thethrottle.
 6. A fuel system as claimed in claim 5 wherein the saidpressure drop means includes engine driven flyweights arranged to applya force proportional to the square of engine speed to the pressure dropmeans to ensure that fuel flow to the engine through the main throttlegenerates a pressure drop proportional to the square of engine speed. 7.A fuel system as claimed in claim 6 wherein the auxiliary fuel flowcontrol comprises an auxiliary variable throttle connected in parallelwith the said main variable throttle, and auxiliary adjustment means forthe auxiliary throttle responsive to the air flow rate within the ductmeans to adjust the auxiliary throttle so that the extra fuel flow tothe engine permitted by the auxiliary throttle is in accordance with theair flow rate within the duct means.
 8. A fuel system as claimed inclaim 7 wherein the said means responsive to air flow within the ductmeans comprises a pair of opposed bellows, one connected to receiveengine compressor delivery pressure and the other connected to receivepressure from the duct means whereby the bellows generates a forceproportional to pressure drop between the engine compressor and the ductmeans, and servo means for adjusting the auxiliary throttle inaccordance with said force.
 9. A fuel system as claimed in claim 7including an engine-driven flyweight device driven at a speedproportional to engine speed, manually-adjustable spring means acting onthe flyweight device to oppose centrifugal movement, manually-operablemeans for adjusting the load of the spring means, second adjustmentmeans for the main throttle responsive to centrifugal movement of theflyweight device, discriminator means responsive to the first and thesecond adjustment means to connect the adjustment to adjust the mainthrottle which will result in the lower fuel flow to the engine.